1. Field of the Invention
The present invention relates to an arylphenoxy catalyst system for producing ethylene homopolymer or copolymers of ethylene and α-olefins. More particularly, the present invention pertains to a group IV transition metal catalyst expressed by Formula 1, a catalyst system which includes the arylphenoxy-based transition metal catalyst and an aluminoxane cocatalyst or a boron compound cocatalyst, and a method of producing an ethylene homopolymer or copolymers of ethylene and α-olefins using the same. In the transition metal catalyst, a cyclopentadiene derivative and an arylphenoxide as fixed ligands are located around a group IV transition metal, arylphenoxide ligand is substituted with at least one aryl derivative and is located at ortho position thereof, and the ligands are not crosslinked to each other.

In Formula 1, wherein M is the group IV transition metal of the periodic table;
Cp is cyclopentadienyl group capable of forming an η5-bond along with the central metal, or derivatives thereof;
R1, R2, R3, R4, R5, R6, R7, and R8 of the arylphenoxide ligand are independently a hydrogen atom, a halogen atom, a C1-C20 linear or nonlinear alkyl group arbitrarily substituted with one or more halogen atoms, a silyl group which contains the C1-C20 linear or nonlinear alkyl group arbitrarily substituted with one or more halogen atoms, a C6-C30 aryl group arbitrarily substituted with one or more halogen atoms, a C7-C30 arylalkyl group arbitrarily substituted with one or more halogen atoms, an alkoxy group which contains the C1-C20 alkyl group arbitrarily substituted with one or more halogen atoms, or a C3-C20 alkyl-substituted or C6-C20 aryl-substituted siloxy group, where the substituent groups may be arbitrarily bonded to form rings;
X can be independently selected from a group including the halogen atom, the C1-C20 alkyl group which is not a Cp derivative, the C7-C30 arylalkyl group, an alkoxy group which contains the C1-C20 alkyl group, the C3-C20 alkyl-substituted siloxy group, and an amido group which has a C1-C20 hydrocarbon group;
Y is the hydrogen atom, the halogen atom, the C1-C20 linear or nonlinear alkyl group arbitrarily substituted with one or more halogen atoms, the silyl group which contains the C1-C20 linear or nonlinear alkyl group arbitrarily substituted with one or more halogen atoms, the C6-C30 aryl group arbitrarily substituted with one or more halogen atoms, the C7-C30 arylalkyl group arbitrarily substituted with one or more halogen atoms, the alkoxy group which contains the C1-C20 alkyl group arbitrarily substituted with one or more halogen atoms, the C3-C20 alkyl-substituted or C6-C20 aryl-substituted siloxy group, the amido group or a phosphido group which contains the C1-C20 hydrocarbon group, or a C1-C20 alkyl-substituted mercapto or nitro group; and
n is 1 or 2 depending on the oxidation state of the transition metal.
2. Description of the Related Art
Conventionally, Ziegler-Natta catalyst system which includes a main catalyst component of titanium or vanadium compounds and a cocatalyst component of alkyl aluminum compounds has been used to produce an ethylene homopolymer or copolymers of ethylene and α-olefins. However, the Ziegler-Natta catalyst system is disadvantageous in that, even though it is highly active in the polymerization of ethylene, the molecular weight distribution of a resultant polymer is wide, and particularly, a compositional distribution is non-uniform in the copolymer of ethylene and α-olefin due to heterogeneous catalyst active sites.
Recently, the metallocene catalyst system which comprises a metallocene compound of a group IV transition metal in the periodic table, such as titanium, zirconium, or hafnium, and methylaluminoxane as a cocatalyst has been developed. Since the metallocene catalyst system is a homogeneous catalyst having one kind of catalytic active site, it can be used to produce polyethylene having a narrow molecular weight distribution and a uniform compositional distribution in comparison with the conventional Ziegler-Natta catalyst system. For example, EP Pat. Nos. 320762 and 372632, and Japanese Patent Laid-Open Publication Nos. Sho. 63-092621, Hei. 02-84405, and Hei. 03-2347 disclose metallocene compounds, such as Cp2TiCl2, Cp2ZrCl2, Cp2ZrMeCl, Cp2ZrMe2, or (ethylene-bis tetrahydroindenyl)ZrCl2, activated with methylaluminoxane as a cocatalyst to polymerize ethylene at high catalytic activity, thereby making it possible to produce polyethylene having a molecular weight distribution (Mw/Mn) of 1.5-2.0. However, it is difficult to produce a polymer having a high molecular weight using the above catalyst system. Particularly, if it is applied to a solution polymerization process which is conducted at high temperatures of 140° C. or higher, polymerization activity is rapidly reduced and a β-hydrogen elimination reaction is dominant, thus it is unsuitable for producing a high molecular weight polymer having a weight average molecular weight (Mw) of 100,000 or more.
Meanwhile, a constrained geometry non-metallocene catalyst (a so-called single-site catalyst) in which a transition metal is connected to a ligand system in a ring shape has been suggested as a catalyst which has high catalytic activity and is capable of producing a polymer having a high molecular weight in polymerization of only ethylene or in copolymerization of ethylene and α-olefin under a solution polymerization condition. EP Pat. Nos. 0416815 and 0420436 suggest a catalytic system in which a transition metal is connected to cyclopentadiene ligand and an amide group in a ring shape, and EP Pat. No. 0842939 discloses a catalyst in which a phenol-based ligand as an electron donor compound is connected with a cyclopentadiene ligand in a ring shape. However, since the cyclization of the ligands along with the transition metal compound is achieved at very low yield during synthesis of the constrained geometry catalyst, it is difficult to commercialize them.
Meanwhile, an example of non-metallocene catalysts which is not a constrained geometry catalyst and is capable of being used under a high temperature solution condition are disclosed in U.S. Pat. No. 6,329,478 and Korean Patent Laid-Open Publication No. 2001-0074722. The patents disclose a single-site catalyst using one or more phosphinimine compounds as a ligand, having high ethylene conversion during copolymerization of ethylene and α-olefins under the high temperature solution polymerization condition at 140° C. or higher. However, a limited range of phosphine compounds may be used to produce the phosphinimine ligand, and, since these compounds are harmful to the environment and to humans, it might have some difficulties in using them to produce general-purpose olefin polymers. U.S. Pat. No. 5,079,205 discloses a catalyst having a bis-phenoxide ligand, but it has too low catalytic activity to be commercially used.
In addition to the above-mentioned examples, Nomura et al., Organometallics 1998, 17, 2152 discloses the synthesis of a non-metallocene catalyst with a phenol-based ligand and polymerization using the same, in which the substituents on the phenol ligand are limited to only simple alkyl substituents such as isopropyl group. On the other hand, Rothwell, P. et al., J. Organomet. Chem. 1999, 591, 148 discloses an arylphenoxy ligand, but does not suggest the effects of aryl substituent at the ortho-position.